Optical monochromators characterize spectral content of optical signals, such as optical channels within dense wavelength division multiplexed (DWDM) optical communication systems. High signal selectivity and low insertion loss are increasingly important performance parameters of a monochromator as the optical channels within DWDM systems become more closely spaced. For example, signal selectivity of at least 35 dB at 0.4 nanometer offsets from the optical channel's center wavelength is desirable to sufficiently characterize wavelength, power and signal-to-noise ratio (SNR) of optical signals within a DWDM system having a channel spacing of 100 GHz, whereas higher signal selectivity, at least 35 dB at 0.2 nanometer offsets, is desirable for a channel spacing of 50 GHz. Low insertion loss is important for measuring low amplitude noise in SNR measurements of a DWDM system.
Grating-based optical monochromators that use multipass configurations have high signal selectivity. However, grating-based monochromators that are physically compact, such as those using a Littman-Metcalf configuration, typically have high insertion loss which degrades measurement sensitivity and may render the monochromators unsuitable for measuring SNR. Walk-off crystals separate polarization states of optical signals into separate optical beams which enables the polarization states to be aligned to minimize insertion loss, thereby improving the measurement sensitivity of the monochromator. However, optical beams having large diameter are not readily accommodated by presently available walk-off crystals, which reduces illumination area of the grating, in turn decreasing the signal selectivity of the optical monochromator. Accordingly, there is a need for an optical monochromator that has both high signal selectivity and low insertion loss.